The present invention generally relates to fasteners and threaded workpieces, and more particularly to affixing an internally threaded insert within a workpiece so that a threaded fastener may be made up to the workpiece utilizing the threads of the insert.
For a variety of reasons it is desirable to dispose a self-tapping sleeve within a workpiece. Most, but not all of the time, the self-tapping sleeve will have internal threads and will be utilized for thread replacement. For example, if pre-existing threads of the workpiece are damaged, the damaged threads may be replaced with the threads of the insert. One type of internally-threaded insert is self-tapping, such that the insert may be driven into a bore of the workpiece, cutting threads in the bore as the insert is driven. The self-tapping inserts have both internal threads for receiving a fastener and external threads. A first group of external threads cuts new threads in the bore, and a second group of external threads makes up into the new threads, thereby advancing and securing the self-tapping insert within the bore and thus providing new threads within the workpiece.
The most common use of self-tapping inserts is to provide replacement threads where the original threads of the workpiece have become damaged, and to stabilize the matrix material to prevent cracks from continuing or propagating. When the original threads become damaged, they can sometimes be repaired by chasing the damaged thread with a tap to restore the original thread shape. However, if the original thread shape cannot be restored by this measure, the thread must be replaced. One means of replacing the threads is to bore the hole to a larger diameter than the original thread diameter and to rethread the hole. However, a disadvantage of this procedure is that it requires a change in the fastener size from the original. If the equipment utilizes multiple fasteners of the original size, the different size fastener complicates maintenance and repair of the equipment because different tools are required, and correlating fasteners with the matching threads made more difficult. It is therefore desirable in some cases to be able to replace the original threads with threads of the same size so that the same size fastener may be utilized. In these cases, self-tapping inserts may be employed.
Self-tapping inserts are hardened steel cylinders, threaded on the exterior and, usually, in the interior. The interior thread diameter and pitch of the internal threads are those of the fastener to be installed. The exterior of the self-tapping insert comprises a section which cuts new threads (the “cutting section”) and a section of threads which make up into the newly cut threads. The cutting section of the known self-tapping inserts is tapered and usually comprises three or more slots or holes, which interrupt the tapered threads, thereby forming teeth similar to those of a conventional thread tap. Typically a bolt (the “drive bolt”) is used to drive the self-tapping insert into a pilot hole in the base metal. This pilot hole is usually made by drilling out the damaged threads as described above to form a bore hole in the base metal. As the insert is turned, the teeth of the cutting section engage and remove the base metal until the insert is fully installed and flush with the exterior surface of the workpiece. The insert remains in place within the workpiece by an interference fit between the newly cut threads in the workpiece and the exterior threads on the insert.
While in a well-equipped shop it is possible to correctly install known varieties of self-tapping inserts with shop equipment such as mills and drill presses, field installation of such devices with hand tools presents several disadvantages for the known self-tapping inserts. Because the known self-tapping inserts are tapered on the tapping end (i.e., the end which is first inserted into the bore hole), the inserts have a tendency to start tapping crookedly. The person performing the tapping procedure has no simple way other than by visual inspection to ascertain whether the insert is entering the bore hole straight—i.e., whether the longitudinal axis of the insert coincides with the longitudinal axis of the bore hole. The only way to ensure that the prior-art insert enters the bore straight is to utilize a magnetic drill (“mag drill”) which attaches to the work-piece with an electromagnet. An example of such a mag drill is disclosed in U.S. Pat. No. 3,969,036 (Hougen). The procedure requires: (1) positioning the mag drill by means of a centering tool; (2) replacing the centering tool with a drill bit and drilling out the damaged threads; (3) customizing the drive bolt by removing its head so that it can be fitted to the chuck of the mag drill; (4) with the mag drill maintained in exactly the same location as established in step (1), threading the prior art insert onto the modified drive bolt and installing the modified drive bolt into the chuck of the mag drill; (5) driving the prior art insert two to three rotations with the mag drill, until it has started to cut new threads; and (6) completing the installation with a wrench, socket wrench, pneumatic impact wrench, mechanical torque multiplier, or hydraulic torque multiplier, depending upon the torque required to install the prior art insert.
It is important that the insert be installed straight, which means it must be correctly aligned at the initiation of the installation procedure. If the insert is too crooked during installation, the insert may shatter when partially installed because of the hardness of the insert. If the insert is installed crooked and does not shatter, the fastener will often not align correctly with the insert. The alignment problem becomes more severe for larger inserts. In recognition of this problem, one manufacturer of self-tapping inserts requires that the installation method for larger diameter inserts (such as larger than ¾ inch) include counter-boring or partially pre-tapping the pilot hole for the insert such that the insert will be properly aligned within the hole. Counter-boring or pre-tapping the pilot holes are demanding, time-consuming and expensive procedures requiring large-diameter drill bits and/or taps, often under difficult field conditions.
A need therefore exists for a self-tapping insert which aligns itself correctly to the axis of the workpiece's borehole, without the need to counter-bore or pre-tap, and without the aid of magnetic drills, mills, drill presses, vises, collets, or similar devices.